The Morphological Basis of the Sign of Synaptic Action. The cholinergic interneuron, cell L1O, a command element in the neuronal circuit controlling circulation, mediates chemical synaptic inhibition to some cells, excitation to others, and dual excitation-inhibition to a third class of cells. I plan first to compare the ultrastructure of excitatory, inhibitory and mixed action terminals of the same neuron in order to study morphological correlates of these differences in function. The Morphological Basis of a Behavioral Modification. The monosynaptic connections between identified mechanoreceptor sensory neurons and the motor cell L7 undergo plastic change in synaptic efficacy that mediates short-term and probably long-term behavioral habituation. I plan to investigate the morphological correlates of the synaptic changes accompanying both the short-term and the long-term habituation, and of the transition from one form to the other, for the purpose of detecting structural differences which might contribute to an understanding of the basic mechanism of behaviorally relevant synaptic plasticity. Since habituation is a useful tool for studying memory mechanisms, this analysis could provide some beginning insights into the morphological processes that underly a form of short-term memory and the conversion from short-term to long-term memory. BIBLIOGRAPHIC REFERENCES: Thompson, E.B., J.H. Schwartz and E.R. Kandel. 1976. A radioautographic analysis in the light and electron microscope of identified Aplysia neurons and their processes after intrasomatic injection of H3-L-fucose. Brain Research (in press).